A filter is a frequency-discriminating device, circuit or network designed to select or pass certain bands of frequencies with low attenuation and to highly attenuate other frequencies. Filters are ubiquitous; they are used in many different types of electrical systems. One important type of system in which filters are used extensively is telecommunications systems. Filters will be found in virtually all elements of telecommunications systems, including, for example, radios, amplifiers and wireless and wireline terminals (e.g., telephones, videophones, facsimile transmission devices) to select certain frequencies in preference to others.
Filters may be categorized according to their characteristic operation (e.g., low-pass, high-pass, band-pass). A band-pass filter, for example, passes a band or bands of frequencies and attenuates all frequencies outside of the "pass" regions. An important property of a filter is its "selectivity." As an illustrative example of that property, consider a radio tuned to the frequency of a particular transmitting station, say 102.7 MHz. While signals from other broadcasting stations having other frequencies (e.g., 101.5 MHz, 104.3 MHz, etc.) are present at the radio's antenna at nominally equal strength to the 102.7 MHz signal, such other signals produce little or no signal strength in the radio.
The frequency (102.7 MHz in the above example) generating a maximum response (e.g., current) in the filter is referred to as the "resonant" frequency. A filter is not infinitely selective; there will be a range of frequencies centered about the "resonant" frequency that are only moderately attenuated. The term "bandwidth" is used to refer to that range of moderately-attenuated frequencies. The selectivity of a filter is typically expressed as a ratio of resonant frequency to the bandwidth. That ratio is referred to as the "quality factor" (Q) of the filter. The greater the magnitude of Q, the greater the selectivity of the filter.
There are several limitations or drawbacks to using conventional band-pass filters comprised of resistors, inductors and capacitors. One drawback is that the quality factor of such conventional band-pass filters is typically limited to a maximum value on the order of 10.sup.2. In many applications, a band-pass filter having a significantly higher Q would be desirable. Another drawback pertains to the limited extent to which such filters can be miniaturized. Currently, such filters have dimensions on the order of inches (e.g., 1".times.1/2"). The size limitation arises due to the phyical geometry of the elements comprising such a filter. Specifically, the relevant properties of inductors and capacitors (i.e., inductance and resistance) is related to their physical size.
It would be desirable to reduce the size of such filters so that associated electronics (e.g., a radio) could be further miniaturized.